Many thin image display devices as typified by displays, i.e., flat panel displays, have been released on the market, because they are thinner than cathode-ray tube displays and they do not occupy much space in depth. Their market price has decreased year by year with advances in production techniques, resulting in a further increase in demand and a yearly increase in production. Especially, color LCD TVs have almost become the mainstream of TVs. Also in recent years, organic light-emitting display devices such as organic EL displays, which emit light by themselves and thereby have high visibility, have received attention as the next generation of image display devices. In relation to the performance of these image display devices, there is a strong demand for a further increase in image quality, such as an increase in contrast and color reproducibility, and a decrease in power consumption.
A color filter is used in these liquid crystal display devices and organic light-emitting display devices. For example, in the case of color LCDs, the amount of light is controlled by using a back light as the light source and electrically driving the liquid crystal. Colors are represented by the light that passed through the color filter. Accordingly, the color filter is indispensable for color representation in LCD TVs and plays a large role in determining display performance. In organic light-emitting display devices, a color image is formed in the same manner as liquid crystal display devices, when the color filter is used in combination with an organic, white light-emitting element.
A recent trend is that there is a demand for power-saving image display devices. To increase backlight use efficiency, there is a very high demand for high-luminance color filters. This is a major issue especially for mobile displays such as mobile phones, smart phones and tablet PCs.
Even though technological advances have increased battery capacity, there is still a limit on battery capacity of mobile devices. Meanwhile, there is a trend that power consumption has grown with the increase in screen size. Image display devices containing a color filter determine the design and performance of mobile terminal devices, because they are directly linked to the usable time and charging frequency of mobile terminal devices.
In general, a color filter has a transparent substrate, color layers made of color patterns of the three primary colors (red, green and blue), and a light shielding part formed on the transparent substrate so as to define each color pattern.
To form such color layers, a pigment dispersion method in which pigments with excellent heat resistance and light resistance are used as color materials, has been widely used. However, it is difficult for color filters produced by use of pigments to satisfy the latest demand for higher luminance.
As a means to achieve higher luminance, color resin compositions for color filters, which contain dyes, have been studied. Compared to pigments, dyes generally have a higher transmittance and are able to produce a high-luminance color filter. However, dyes are problematic in that they are inferior in heat resistance and light resistance to pigments and the chromaticity is likely to change when they are heated at high temperature in color filter production process, for example. Also, color resin compositions containing dyes have such a problem that aggregates are likely to be precipitated during drying process. Aggregates precipitated in a coating film lead to a remarkable deterioration in contrast and make it difficult to use the coating film as a color layer.
As a means to improve various kinds of resistance properties of dyes, a method for producing a salt-forming dye is known.
In Patent Literature 1, a color composition for color filters is disclosed as a color resin composition for color filters which is excellent in color characteristics, heat resistance, light resistance and solvent resistance, the color composition containing a salt-forming compound formed from a basic dye and an anion component having a molecular weight of 200 to 3,500. However, a color layer using the color composition for color filters disclosed in Patent Literature 1 shows insufficient heat resistance during a high-temperature heating step in a color filter production step.
In Patent Literature 2, a salt is disclosed as a dye with an excellent molar absorbance coefficient, the salt being formed from a cation having a xanthene skeleton and an anion having a triphenylmethane skeleton. In Patent Literature 2, the salt is described to be soluble in organic solvents, and it is not dispersed in resin compositions for use.
A color filter and so on are disclosed in Patent Literature 3 by the inventors of the present invention, which use specific color materials containing divalent or higher anions and divalent or higher cations, in which dye skeletons are crosslinked by crosslinking groups. It is disclosed that the color materials are excellent in heat resistance since, due to containing the divalent or higher anions, molecular associations are formed therein, and color filters using the color materials have high contrast and are excellent in solvent resistance and electric reliability.